How to use an op amp

An op amp is one of the basic building blocks in electronics and learning how to use an op amp will help you advance in this fascinating hobby. Although an op amp is a fairly complex integrated circuit containing quite a few transistors and passive components, understanding how it operates and how to use it isn’t.

If you’ve read my post what is an op amp, you will have some knowledge of the theory however this post is more for practical examples. If you are anything like me, reading stuff isn’t the best way for me to learn. It tends to go in one ear and out the other, however, I tend to remember much more if I’ve done something so that is how I’m going to show you how to use an op amp.

How to use an op amp

Voltage follower

This is the first and probably the most simple op amp circuit. It’s known as a voltage follower or buffer. If you connect a signal generator to the input you will get the same signal on the output as on the input.

What use is that I hear you ask? Have you ever connected the output of something to the input of something else and it’s not worked as you had hoped or expected?

What use is a voltage follower?

That may sound a bit confusing so I’ll use an example. I built something like an oscillator to produce an output waveform. I wanted it to control another circuit that I had built. It didn’t alter the circuit as I had thought.

I connected it to an oscilloscope so I could see it was producing the required waveform output but when I connected it to my other circuit the output dipped to practically zero. As I disconnected it from the other circuit’s input the signal returned.

I was puzzled as to what was happening. It worked fine on its own but connected to the other input and it stopped.

You might already have guessed but I was inexperienced in electronics at the time.

The other circuit was clearly affecting the output of my oscillator. It was because the input of the other circuit had a low impedance. This was “drowning” the weak output of my oscillator.

I didn’t know at the time but what I needed was a voltage follower. Just like we built at the beginning of this paragraph.

That is the use of a voltage follower. The signal coming out of the oscillator wasn’t enough to drive the input stages of the other circuit.

The voltage follower has a high impedance so it’s very easy to drive with a weak signal. The op amps output will drive something following it with a low impedance. Although it doesn’t provide a larger voltage signal it provides a more powerful one by being able to drive the following input stage with more current.

Inverting amplifier

This is another of the op amp circuit building blocks. It amplifies and inverts the signal at the same time.

how to use an op amp unity gain inverter

In the circuit above the gain is = (R2 / R1)

Which is 1 / 1 = 1 so with a gain of 1 it isn’t really an amplifier, it’s just inverting the signal. However, if we increase the value of R2 to 2K it becomes:

2 / 1 = 2 so a gain of 2.

Increase the value of R2 to 10K and it becomes:

10 / 1 = 10. You need to bearing mind though that the value of the output voltage can only go to the same (a bit less actually in practice) as the supply voltage. So with a gain on 10 and an input of 5V you won’t get 50V out.

Non inverting amplifier

This op amp circuit just amplifies without inverting.

This time though the formula for gain is:

Gain = 1 + ( R2 / R1 )

So in the above circuit the gain is 1 + ( 1 / 1 ) = 2.

Increase R2 to 10K and we get 1 + ( 10 / 1 ) = 11.